1. Field of Invention
The invention relates to a method and apparatus for generating and sensing an electromagnetic field defining a wireless boundary. More specifically, this invention relates to a method and apparatus for determining the proximity of a receiver to an electromagnetic field boundary generated by a wireless transmitter, especially for animal containment.
2. Description of the Related Art
The present invention relates in general to proximity monitoring systems for determining when a second device (i.e., receiver module) is proximate to a first device, (i.e., transmitter), (which is the functional equivalent to determining when a second device is proximate to a wireless boundary encompassing and defined relative to the location of the first device) and is particularly directed to a high sensitivity, low cost proximity detection system which employs a modulated, quasi-static magnetic field and requires a small, very low cost and very low power second device as in a wireless pet containment application.
Proximity detection devices are used in a variety of applications for determining the relative nearness of an object, animal or person to a designated area or location or to the location of another object or person. One important area of application would be to determine if a child strays too far away from a certain location or from a parent or guardian. Another application would be to determine if an institutionalized individual or a tagged equipment item has strayed or been carried away from a designated area. Still another important area of application would relate to determining when a device is proximate to a kiosk for the purpose of establishing wireless communications only when the proximity is a prescribed distance. Still another important application would be for a pet containment system where a device worn by the pet must self-detect when it is proximate to a fixed wireless boundary.
Prior art methods for these types of proximity detection applications can generally be classified according to whether an implementation of the proximity detection method requires the second device (typically the portable, mobile device) to have both transmit and receive functionality or whether the method can be implemented with a receive-only second device. Examples in the prior art of systems requiring both transmit and receive functionality in the second device include those methods which rely on the transit time or phase shift properties of an ultrasonic or radio frequency signal. One major disadvantage of all such prior art methods is the relatively low battery life resulting from the relatively high power dissipated when the device is transmitting. Other major disadvantages are the relatively higher cost and larger size required for implementing transmit and receive functionality as compared to implementing a receive only device.
Proximity detection methods which can be implemented with a receive only second device can generally be sub-classified as systems which determine proximity by detecting the received signal strength of a propagating radio frequency signal (typically above a few hundred kHz) or systems which determine proximity by detecting the received signal strength of quasi-static, low-frequency (below 500 kHz and more typically below 100 kHz) magnetic field signal. The accuracy and repeatability of proximity detection based on radio frequency received signal strength is generally known in the prior art to be severely affected by multipath reflections from stationary or mobile reflecting surfaces and by field distortion variations caused by antenna proximity to conductors such as body tissue. Consequently these kinds of methods are not generally suitable for most proximity detection applications including wireless pet containment.
One prior art proximity monitoring system based on quasi-static magnetic fields is the wireless pet containment method of Weinberg which employs a stationary, unmodulated 1-axis magnetic field generator and a pet-worn receiver that requires a multiplicity of 1-axis sensing antennas and a corresponding multiplicity of single conversion receivers to form a measure of the incident magnetic field that is substantially independent of the receiver orientation. Major disadvantages of this method are the increased receiver circuit complexity, cost, size and battery current associated with the requirement for a multiplicity of sensors and receiver channels. Also, this method for detecting the signal strength of an unmodulated carrier cannot achieve very low noise bandwidths needed for maximum receiver sensitivity unless tightly matched and expensive oscillator crystals are used in both the signal generator and receiver. Another proximity monitoring system based on quasi-static magnetic fields is the child monitoring method of Belcher which employs a plurality of orthogonal magnetic fields modulated in a time sequential fashion using on-off amplitude keying at rates in excess of 1 kHz. This method also requires a multiplicity of receiver sensor antennas and a corresponding multiplicity of receivers to achieve an orientation-independent proximity detection performance. It therefore suffers the same disadvantages of increased complexity, size, battery current and cost. Also the relatively fast response time required to amplitude demodulate the on-off keyed carrier is not compatible with achieving the very low noise bandwidths needed to maximize receiver sensitivity.
There remains a need for a proximity monitoring and wireless pet containment system based on low-frequency magnetic fields and having minimum receiver size and cost in addition to maximum receiver sensitivity and battery life.